The present invention relates to integrated circuit devices and, more particularly, to gate structures of integrated circuit devices and methods of forming the same.
It is known to provide gate structures for MOS field effect transistors using a polysilicon layer gate and a silicon dioxide gate dielectric between the polyilicon layer and the underlying semiconductor substrate. However, MOS field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. If the thickness of the silicon dioxide gate dielectric is less than about 40 angstroms (Å), direct tunneling may occur through the gate dielectric to the underlying channel region, which may increase leakage current and power consumption.
Forming the gate dielectric from certain high-k dielectric materials, in place of silicon dioxide, can reduce gate leakage. For high dielectric constant (high-k or high-∈) materials used for the gate dielectric layer, the high permitivity (∈) of the material generally results in an increase in the gate-to-channel capacitance. The capacitance is generally characterized as C=∈A/tox. As such, a gate dielectric having a higher ∈ may allow use of thicker gate dielectric (i.e., tox greater than 40 angstroms) and greater capacitance and device speed may be provided with less leakage current.
One potential disadvantage of using high-k dielectric materials for gate dielectric layers in integrated circuit (IC) devices is that the high dielectric materials generally contain a greater number of bulk traps and interface traps than thermally grown silicon dioxide (SiO2). These traps typically affect threshold voltage (Vt) operation of the transistor. In addition, high gate depeletion of a polysilicon gate electrode may result compared to a silicon dioxide or silicon oxynitride (SiON) gate dielectric. In addition, there may be degradation of bias temperature instability (BTI) characteristics of the transistor and mobility degradation as compared to the silicon dioxide or silicon oxynitride gate dielectric structure. With respect to BTI characteristics of a transistor, negative BTI (NBTI) for a PMOS transistor or positive BTI (PBTI) for a NMOS transistor provide a reliability test for thin film quality.